Ammoniacal leaching systems for the recovery of metals from ores such as laterites or sulphides have been known for some time. Nickel, copper and cobalt form ammine complexes in ammonia-bearing solutions, whereas iron and some other metals do not. This feature is exploited to effect a separation of metal valves from unwanted species in such ores by leaching with an ammonia-bearing solution. Recently, for example, attention has been directed to treating deep sea manganese nodules by ammonia leaching to selectively recover the contained copper, nickel and cobalt while leaving as residue the less valuable manganese that makes up the bulk of these ores.
Recovery of the dissolved nickel and copper from the ammoniacal leach solutions can be accomplished by a variety of well-documented means including, for example, steam stripping or pressure reduction with hydrogen. This invention is primarily directed however, to a recently developed field of art in which the metal values are recovered by solvent extraction with organic reagents and the resulting raffinate (barren leach solution) is recycled to the leaching operation. Cobalt-containing ores present a serious problem to progress in this art because cobalt can build up either in the aqueous or organic phases depending upon its valence.
It is well-known that divalent cobalt in ammoniacal solution is co-extractable with copper and nickel by many organic reagents but, unfortunately, the organic holds the cobalt so tenaciously that, either it cannot be removed, or, requires such severe stripping conditions as to make the process impracticable. It is more usually the case that cobalt is present as a trivalent species in ammoniacal solutions as a result of the oxidizing conditions used in ammonia leaching. Under these conditions it is equally well-known that cobalt will not extract into most organic reagents during copper and nickel extraction but remains behind in the ammoniacal raffinate. In either case, therefore, cobalt builds up in the process cycle. If it builds up in the organic phase the active reagent sites therein are poisoned and the ability of the reagent to pick up copper and nickel is progressively reduced. Alternatively, a cobalt build-up in the ammoniacal solution gradually decreases the leaching power of the raffinate each time it is recycled. In this regard, evidence is available that the presence of cobalt in ammoniacal solutions, even in small quantities, can adversely affect the leaching of metallic nickel from certain ore materials.
Most attempts in the art to solve the cobalt problem have centered on methods to selectively extract cobalt in the presence of nickel and copper and then selectively elute the loaded organic phase to recover nickel preferentially. Typical of such processes are those described in U.S. Pat. Nos. 3,438,768 and 3,728,366. In both of these patents, however, the selectivity of the extraction or elution operations is inadequate, and additional steps, such as selective scrubbing or secondary elution of the loaded organic reagent, are required to generate cobalt eluates that are acceptably low in nickel content. Apart from the general lack of adequate selectivity evident in these methods, each method has additional specific disadvantages that include severe elution conditions, high reagent costs and high solubility of the organic phase in the ammoniacal solution. These and similar processes are held as being evidently unsatisfactory on the grounds that they have not yet found commercial application.
In contrast, a commercially used process does exist that provides a quantitative separation of cobalt (in the trivalent state) from nickel and/or copper. The process is described by Drobnick et al. in U.S. Pat. No. 3,276,863 and employs as extraction agents alpha-hydroxy oximes sold commercially by General Mills under the trade name LIX. Copper and nickel are extracted and cobalt is left behind as a dilute species in the raffinate phase. Unfortunately, means are not yet known whereby the cobalt can be satisfactorily recovered from the raffinate before the latter is recycled to the leach. To date, in the absence of any workable solvent extraction method for cobalt, cobalt recovery from these raffinates is based on precipitation processes such as precipitation of cobalt sulphide using hydrogen sulphide or, in another method, the precipitation of cobalt carbonate by steam stripping ammonia from the raffinate. The former method produces a precipitate with poor handling properties and a tendency to sulphation by air, and also introduces undesirable sulphide ions into the solution. In the latter process, steam stripping huge quantities of ammonia from barren solutions to recover the small amount of contained cobalt is simply uneconomic.
It is apparent from this discussion that adequate means do not yet exist to recover cobalt from ammoniacal solution by solvent extraction. It is felt that part of the reason for this is the complexity of cobalt ammine chemistry, many areas of which are still the subject of theoretical debate. Cobalt forms a great variety of ammine complexes, many of which usually co-exist in any given ammoniacal solution. The prior art has failed to distinguish between these species with respect to their recovery by solvent extraction. U.S. Pat. No. 2,848,322 discloses a method for separating cobalt from nickel by absorbing ammine complexes of both metals onto cationic exchange resins. The inventors, however, point out that several cobalt ammine complexes are probably present in their solutions. The method yields a satisfactory nickel eluate, but significantly, the majority of the cobalt can only be recovered from the resin by soaking in concentrated hydrochloric acid for a period of several hours.
Information concerning specific cobalt ammines is confined almost exclusively to analytical and preparative inorganic chemistry. For example, it is known to prepare cobalt hexammine by oxidizing acid solutions of divalent cobalt in the presence of ammonia and activated carbon. The method is only directed, however, to the preparation of the hexammine for structural studies and the like, and nothing is taught concerning the solvent extraction properties of the ammines.
The present state of the art is best summarized by a very recent reference from the May 1976 issue of "Hydrometallurgy". In an article beginning page 319, it is shown that a method for converting cobalt in ammoniacal solution to the hexammine complex prior to its absorption on the resin would be highly desirable. The article points out, however, that recent tests aimed at generating the complex were not encouraging and a pressurized ion exchange technique would be required, either alone or in combination with the precipitation methods described hereinbefore.
A process has now been discovered that recovers cobalt by ion exchange means. The method does not require elevated pressures and can be applied to a wide variety of cobalt-bearing ammoniacal solutions, including those which also contain nickel and copper. A cobalt ammine complex is extracted that is readily eluted from the organic phase to produce a cobalt eluate of high purity.